how to add date into TreeSet and add them up - hashmap

Question

Im currently working on my assingment which are WeatherData to process 100 years of weather data from a single weather station.
Im working on to get total Snowfall for Year. But i have no ideal how to create hash table where the key is year and the value the sum of all snowfall that year
public WeatherData(Scanner file){
highTemps = new TreeSet<>();
lowTemps = new TreeSet<>();
annualSnowFall = new HashMap<>();
annualSnowFall = new HashMap<>();
highTemps = new TreeSet<>();
lowTemps = new TreeSet<>();
//advance the scanner past the header row
file.nextLine();
while (file.hasNext()) {
// First we split the row by commas
String[] row = file.nextLine().split(",");
// then we remove the quotation marks from each string
for (int i = 0; i < row.length; i++) {
if (!row[i].isEmpty()) {
row[i] = row[i].substring(1, row[i].length() - 1);
}
}
int months = Integer.parseInt(row[DATE].slit("-")[1]);
if (s.length > SNOW && !s[SNOW].isEmpty()) {
double snow = Double.parseDouble(s[SNOW]);
// Then there's snow data for that day
if (!annualSnowFall.containsKey(month)) {
annualSnowFall.put(month, 0.0);
}
// // add the snow we just found to that years
double current = annualSnowFall.get(month);
annualSnowFall.put(month, current + snow);
}
}
}
public double totalSnowfallForYear(int year) {
try {
return (double) annualSnowFall.get(year);
} catch (Exception NullPointerException) {
return 0.0;
}
}